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Bichel Steel for 
Crank * pirn 
anb Biles 



m 
lb. ]f. 3. porter, fiD. le. 



IReprlnteO from 

Zbe IRailwaB an& jEngfneeting ■Review 

aprll te, 1898 



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NICKEL STEEL FOR CRANK-PINS AND AXLES.* 

By H. F. J. Porter, M. E. 

It hardly seems necessary tO' explain to an engineer why, 
if he wants material to sustain severe usage in the nature of 
alternating stresses, he should select a material possessing 
a very high elastic Hmit. And yet it is not unusual to find 
engineers — particularly railroad engineers — who prefer to 
use wrought iron for their locomotive crank- and crosshead- 
pins and axles in preference to steel, because, as they say, 
''steel being crystalline is brittle and snaps off suddenly under 
such service as that under consideration; while iron, having 
fiber, is tougher and yields before breaking." Most of these 
men know better. They all know that the fiber in wTOUght 
iron is due to the slag which could not be eliminated from 
it in manufacture. It is a common laboratory experiment to 
pass chlorine gas through a glass tube containing a piece of 
wrought iron. The iron is dissolved out, leaving a skeleton 
of slag of the same shape as the piece of iron. Here and 
there in the interior of the skeleton can be seen accretions of 
slag which, imbedded in the original piece of wrought iron, 
would weaken it appreciably. Instead, therefore, of being 
an element of strength, the fiber is evidence of an impurity 
which is a source of weakness. 



* A paper presented at the February, 1898, meeting of the Western 
Railway Club, and subsequently revised by the author. 



We hear of many forgings having failed in the past from a 
mysterious so-called "crystallization from shock" or "vibra- 
tion" in service. We all ought to know that metals crystal- 
lize only in cooling from a fluid to a solid state, and that any 
unusual crystallization shown in a fractured forging was there 
when the forging was made, and was due to improper 
heat treatment during manufacture. 

There is still much to be learned about steel, but there is no 
longer any mystery about the breaks that occur in its use. 
If the elasticity of the metal exceeds by a large factor of 
safety the external stress applied, the life of the material will 
be long. In other w^ords, the stronger the material the 
longer it will last under any given service. The cause of the 
final failure to longer endure the stresses appHed is the gradual 
overcoming of the internal molecular force of cohesion. To 
this gradual weakening of the metal has been given the name 
''fatigue." The endurance of metal to resist fatigue has been 
carefully studied and much valuable information on the sub- 
ject has been collected. 

As far back as 1871 experiments were reported by Woehler 
in Germany, followed by Spangenberg and Martens and 
Bauschinger. In England the subject was similarly reported 
upon by Mr. (now Sir) Benj. Baker, and it has since been 
pursued systematically in this country by Mr. James E. 
Howard at the government testing bureau at Watertown, 
and others. And after all the time and labor that has been 
devoted to the investigation, what information has been de- 
rived? Simply this, that iron and steel are more amenable 
to the known laws of the universe than we had previously 
given them credit for being; that the same laws of nature 
under which "continuous droppings will wear away a stone" 
are applicable to these metals. Repetitions of load in amount 
far below the ultimate strength of the material will eventually 
break down its resistance and cause failure. Beyond this 
statement laws of a general character have been formulated, 
but the complex nature of the situation prohibits exactness. 
In "endurance tests" of this character, Woehler found that 



the rupture of a bar of wrought iron by tension was caused 
by any of the following ways : 

By I application of 55,000 lbs. per. sq. in. 

By 800 applications of 51,500 lbs. per. sq. in. 

By 107,000 applications of 47,000 lbs. per. sq. in. 

By 341,000 applications of 42,500 lbs. per sq. in. 

By 481,000 applications of 38,000 lbs. per sq. in. 

A piece of spring steel, subject to bending, broke as 
follows : 

Under 81,000 applications of 95,000 lbs. per sq. in. 

Under 154,000 applications of 85,000 lbs. per sq. in. 

Under 210,000 applications of 75,000 lbs. per sq. in. 

Under 472,000 applications of 65,000 lbs. per sq. in. 

Under 539,000 applications of 58,000 lbs. per sq. in. 

Under 1,165,000 applications of 53,000 lbs. per sq. in. 

Xo two pieces of metal are alike in chemical composition, 
and if they closely approach similarity in this respect, we now 
know that the difference in mechanical treatment during 
their manufacture may cause them to possess widely dilterent 
physical properties. Speaking generally, however, regarding 
iron and steel, we know that for any given stress a certain 
number of repetitions produces failure, the greater the in- 
tensity of stress the smaller the number of repetitions. We 
know also that the stress required to cause failure is less and 
roughly speaking, only one-half as great when the metal is 
strained alternately in opposite directions, as in crank-pins, 
axles, etc., as where it is strained in one direction only, as in 
eye-bars of bridges. 

Let us submit a bar of steel or iron to 30,000 lbs. per sq. in. 
tensile, or to 30,000 lbs. per sq. in. compressive stress. In 
either case the ''range" is the same, viz. : 30,000 lbs. Now 
let us subject the bar to- 15,000 lbs. per sq. in. tensile and to 
15,000 lbs. per. sq. in. compressive stresses alternately, and 
although the range is 30,000 lbs. as before, the life of the 
material thus strained will be only half as long, although 
neither the tensile nor compressive stress approaches the 
elastic limit of the metal as closelv as in the first two cases. In 



other words, the more the metal is maltreated, the shorter 
the time it will endure. But if w^e can make the "range" of 
stress low enough, a practically unlimited number of repeti- 
tions is required to cause failure. It is in fact very striking 
how regularly progressive the increase in the number of 
repetitions is as the range of stresses decreases. 

In the apparatus used to make endurance tests of this 
character conditions are imposed on the metal imitating 
those which occur in actual practice, in such machine parts as 
car axles, engine shafts, crank-pins, etc., where the fibers of 
the metal are subject to stresses, continually varying from 
tension to compression. 

It has been found that within a certain limit, which is ap- 
proximately one-half of the ultimate strength, the metal is 
elastic; and that if strained beyond this point its working 
strength is exceeded and it can no longer be depended upon 
to sustain even minor loads. Such tests give results, how- 
ever, which are simply relative. Their actual significance is 
uncertain. The fact that a miCtal possesses a certain elastic 
limit, elongation and contraction of area when ruptured by 
once loading, fails to convey an adequate idea of what the 
same metal will do under circumstances of repeated stresses, 
or when these stresses are applied in alternate directions, as 
they are in practice. These "endurance tests," therefore, 
have been made in connection with the usual standard tests 
to determine the relations that exist between the two. From 
a careful comparison of these relations, knowledge is obtained 
so that through a determination of the qualities of metal by 
the cheap and rapid standard tests a prediction can be made 
of the conduct and endurance of the same metal in an actual 
service, analogons to that in the endurance test, which is a 
long and expensive one, and therefore, impracticable for use 
in commercial work. 

Following are the results of tests on bars broken from time 
to time during the past few years at the Watertown arsenal. 
11ic records of the wrought-iron l)ars are the average of a 
lan>c numl)er of tests: 



35,000 


30,000 


Rev's. 


Rev's. 


175,000 


625,000 


765,000 




5,100,000 




9,600,000 




15,000,000 


12,550,000 


19,000,000 


16,300,000 




50,000,000 




(Not 




ruptured.) 



5 



Fiber stress, lbs. per sq. in., 40,000 

Rev's. 

Wrought iron breaks after 59,ooo 

. 15 per cent, carbon steel breaks after. . 170,000 
.25 per cent, carbon steel breaks after.. 229,000 
•35 per cent, carbon steel breaks after. . 317,000 
.45 per cent carbon steel breaks after. . 976,000 

•55 per cent carbon steel breaks after 

.65 per cent, carbon steel breaks after. . 3,689,000 

.75 per cent, carbon steel breaks after 

.85 per cent, carbon steel breaks after 

.95 per cent, carbon steel breaks after 

1.05 per cent, carbon steel breaks after 



3% per cent, nickel steel, carbon .25 to 

.30 per cent 1,850,000 

4% per cent, nickel steel, carbon ,25 to 

.30 per cent., 2,360,000 

53^ per cent, nickel steel, carbon .25 to 

.30 per cent., 4,370,000 



These results, which are representative of many, show that 
the material after a certain number of repetitions of stress 
within the elastic limit breaks with fewer subsequent repeti- 
tions. It is impossible not to conclude, whatever the cause 
of decreased life of the bar may be, it is a cause which acts 
continuously, altering in some way its structure or properties. 

It would naturally appear likely that any gradually pro- 
gressive alteration or "fatigue" of the bar would be mani- 
fested in some way in alteration of the ultimate strength, 
elastic limit or elongation of the metal, when tested in the 
ordinary way. This, however, does not appear to be the case. 

Careful consideration of the results of endurance tests so 
far made, leads to the recommendation of material for forg- 
ings, if they are to be subjected to frequently alternating 
stresses, which shall have a very high elastic limit and that 
they shall be so proportioned that these stresses shall at all 
times lie far within this limit. Steel, not wrought iron, 
should be the metal used, and the higher the carbon content 
of the metal the higher will be this limit, and the longer will 



be the life of the piece. A low percentage of nickel in its 
composition will increase its life still further. 

High-grade steel forgings, sound and free from internal 
strains, are by no means easy to produce. Complete 
chemical, physical, metallurgical, and microscopical labora- 
tories should be an adjunct of every forge that expects to 
do conscientious work. The forge should possess its own 
steel plant and cast its own metal. The chemical composi- 
tion of the metal from which the forging is to be made should 
be carefully considered and specified, and tests from the 
ingot metal should show that what was specified has been 
obtained. Records should be kept of the treatment of every 
forging as it progresses from each successive stage to 
another. The ingot should be cooled from its fluid state, 
until it is solid, under sufficient pressure to expel air and 
gases. The reheating of the ingot for forging should be 
carefully supervised to prevent the surface metal expanding 
away from the center and forming cracks from too rapid 
heating. The forging process should be conducted under a 
slow-working hydraulic press instead of a hammer, in order 
to insure thorough flowing of the metal. After the forging 
process is completed, subsequent heat treatment must follow 
to relieve the metal of forging and cooling strains. Let us 
consider the ''rationale" of this heat treatment a little more at 
length. 

If we note the rate oi cooling of a steel ingot from the 
point of solidification to coldness (see curve), we will see that 
the temperature will fall with a gradually retarded rate until 
between 1300 and 1200 degrees Fahrenheit a point (depend- 
ing on the carbon content) is reached where the temperature 
suddenly stops falling and for a time either remains stationary 
or perhaps rises for a short time, and then the rate of cooling 
continues as before. This point, where the change of rate 
takes place, is called the ''recalescent" point, and from chemi- 
cal and physical tests we know that a change in the structure 
of the steel occurs here. 



The fluid steel begins to crystallize at the point of solidifi- 
cation, and the slower the rate of cooling from there down the 
larger the crystals will be when the ingot is cold. At the 
point of recalescence, however, it would seem as if the crystal- 
lization, so to say, locks itself, for after the ingot has become 
cold, if we reheat it to a temperature below this point, on 
again becoming cold we will find that the crystallization is 



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13 14 15 16 



THE COOLING CURVE OF STEEL. 



not aft'ected, but if we reheat it a little above the recalescent 
point, when it is again cold the crystallization will be found 
to be smaller than before. 

In fact it is known that if steel is heated slightly above the 
recalescent point, all previous crystalHzation is destroyed and 
a fine amorphous condition is produced at that temperature. 
As soon as cooling begins again, crystallization sets in and 
continues until the ingot is cold. 



As, however, the time of 



8 



cooling- irom the recalescent point is comparatively short the 
resultant crystallization is correspondingly small. It can be 
readily understood that when heat treatment can completely 
change the internal condition of steel, it should bear an im- 
portant part in the manufacture of forgings made of that 
metal. 

Let us for a moment consider the changes which take 
place in the condition of the metal as it passes through the 
forging process. Beginning with the cold ingot (which we 
will assume has cooled slowly and is therefore composed of 
large crystals), we first reheat it up to a forging temperature 
of froim 1800 to 2000 degrees Fahrenheit, thus passing 
through the recalescent point, destroying all crystallization 
and producing an amorphous condition. As we put it under 
the forging press it begins to cool, crystallization at once 
setting in ; at the same time, however, we begin to work the 
metal. 

The work of forging tends to check crystallization, just as 
disturbing water which is below freezing point will delay the 
formation of ice crystals. This work may or may not con- 
tinue (depending upon the size and shape of the finished 
piece) until the temperature has fallen below the recalescent 
point, but during this time more or less crystallization has 
occurred and has been disturbed and distorted. The work 
of forging has, moreover, proceeded from one end of the 
piece to the other, the part last worked upon having crystal- 
lized considerably before work was applied to it, so that the 
two ends may be entirely different as far as their internal 
condition is concerned. 

If, as is generally the case, the forging is now considered 
finished, it is full of pulls and strains about which we know 
nothing except that they may amount to several thousand 
pounds to the square inch. The extent of these strains is 
made evident when a forging, finished as above described, 
has a cut taken from it in a lathe or has a keyway cut on one 
side. The strains in the fil)ers which are cut are relieved, and 
the ])iecc invariably springs out of "true." To relieve these 



strains the forgings should be carefully and slowly heated 
to a temperature slightly above the recalescent point and then 
allowed to cool slowly. By this treatment, which is called 
"annealing," an entirely new crystallization is established, 
leaving the molecules of the metal completely at rest. If the 
forging, on being heated slightly above the recalescent point, 
is suddenly dropped into a bath of cold oil no time is allowed 
during the cooling process for crystals to form, and the amor- 
phous condition of its structure at that temperature is re- 
tained. This character of heat treatment is called "oil 
tempering," and is followed by further heat treatment to re- 
lieve the mictal of any hardening effect due to the cooling 
process. 

In sudden cooling the surface metal of solid forgings is apt 
to shrink on to the metal of the interior to such an extent as 
to crack it open. In order, therefore, to oil-temper a forging 
with safety it should be hollow to allow the heat to be ex- 
tracted from the interior and surface metal equally. 

Annealing lowers the ultimate strength and elastic limit of 
steel, but increases its ductility, as shown by the elongation 
and contraction in test specimens. Oil-tempering not only 
restores the ultimate strength and elastic limit, but increases 
the elongation and contraction very considerably. 

We have seen by the endurance tests above tabulated that 
even the very best steel suitable for forgings will not last in- 
definitely under a fiber stress of 40,000 lbs. per sq. in., and 
that the life of wrought iron and mild steel under even lower 
stresses is comparatively short. It is not surprising, from the 
light shed upon the subject by these tests, that the mortality 
of forgings as usually made for crank-pins, axles, etc., is 
very great. These parts are generally made of material which 
has initially little strength to resist fatigue. They are loaded 
up during the process of forging by stresses which may 
closely approach the elastic limit before any stress whatever 
is put upon them from outside. 

AA'hen it is remembered that, besides these stresses, a severe 
compressive stress, sometimes itself approaching the elastic 



lO 



limit, is applied to the metal by the process of forging them 
into wheels, it is not surprising that their life is short at the 
point where the stress is applied. 

For such service as is required of crank- and cross-head pins, 
axles, piston and connecting rods and all forgings subjected to 
stresses alternating from tension to compression a metal pos- 
sessing the very highest elastic limit possible should be sup- 
plied. We must be sure that our forgings are free from in- 
ternal strains, so that they will be subjected in service to only 
those stresses which we know about and have calculated will 
be applied to them externally. We must also so proportion 
them that these stresses will be well below the elastic limit of 
the metal if we desire to insure lasting qualities, remembering 
that "range" of stress, when the stresses are alternating, has 
a double value. High carbon steel is hard and therefore diffi- 
cult to machine to shape and in order to obtain the quality of 
metal most desirable for the service under consideration 
where a combination of high elastic strength and ductility is 
of the first importance, steel makers have done much experi- 
menting to determine the effect of varying composition, in- 
cluding the introduction of such unusual elements as chrom- 
ium, nickel, tungsten, etc. 

NICKEL STEEL. 

The first practical application of the improvement in the 
quality of steel by the use of nickel was in the production of 
open-hearth armor plate. This alloy increased the toughness 
of steel of a given hardness or tensile strength so^ as to add 
greatly to its resistance to cracking from shock of projectile 
impact. An examination of the physical characteristics of 
this metal shows it to possess valuable qualities which explain 
its toughness and resistance to shock. By replacing some of 
the carbon by a small per cent, of nickel in steel of a given 
composition, the physical properties are modified, the elonga- 
tion being considerably increased and to a still greater degree 
the contraction of area in test pieces at the point of fracture. 

Its effect upon the elastic limit, however, is of the greatest 



1 1 



importance, as it raises this quality in a marked degree rela- 
tively to the tensile strength, and thus insures a combination 
of elastic strength and ductility unknown in any other metal. 

The presence of the nickel also renders the steel sensitive 
to the good elTects of tempering, and the desirable qualities 
above mentioned are accentuated by this treatment. The 
substitution of nickel for a portion of the hardening element 
overcomes the difficulty of machining above referred to. En- 
durance tests show that this metal is eminently adapted to re- 
sist fatigue, and experience in actual practice confirms these 
tests. 

Test bars of this metal one-half inch in diameter and two 
inches long between measuring points, taken from prolonga- 
tions left on the end of forgings for the purpose, show the 
following physical properties : 

Tensile strength, 85,000 to 95,000 lbs. per sq. in. 

Elastic limit, 55,000 to 65,000 lbs. per sq. in. 

Elongation, 24 per cent, to 21 per cent. 

Contraction 45 per cent, to 50 per cent. 

These records are taken from tests made on a carefully 
standardized testing machine, the elastic limit being deter- 
mined by an electric micrometer, not by the drop of the beam, 
which method is approximate only, and might record any- 
where from 2,000 to 10,000 lbs. per sq. in. higher. 

Many railroads are now taking up the use of this metal 
generally, having tested it experimentally and found to their 
satisfaction that, by a small initial increased expense, they 
can save largely by having fewer breaks with their attendant 
delays and damage. 



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